1. Field of the Invention
A general electrostatic image forming apparatus comprises a document reading unit for illuminating a document image and transmitting light beams reflected therefrom to a photoconductor, an image forming unit including the photoconductor on which electrostatic latent images are formed in response to light beams from the image forming unit and the document images are reproduced, and a sheet delivering unit for feeding a copy sheet (a copy medium) to the image forming unit and ejecting the copied sheet on which the document image has been formed.
The document reading unit includes a document table on which an original document, or an original document medium to be read out is loaded, an illuminator for illuminating the original document, and an optical section for transferring light beams reflected from the original document.
The image forming unit comprises the photoconductor which is rotatably provided and on which electrostatic latent images are formed in succession in response to the light beams transferred from the document reading unit, a charger for loading electric charges on the photoconductor, a developing section for developing the latent images formed on the photoconductor, a transferring section for transferring the developed image on a transcript sheet (a transcript medium), a fixing device for fixing the images transferred to the transcript sheet, and a cleaner for cleaning the photoconductor so as to return the same to the initial state.
The sheet delivering unit has a sheet cassette for receiving the sheets to be transferred (the transcript sheets), a sheet feeder for supplying the transcript sheets to the image forming unit, and an output section for ejecting the sheets on which the image has been formed.
In the electrostatic image forming apparatus, a predetermined electric charges is given to the photoconductor, having photo-conductivity by means of the charger. The light beams reflected from the original document form images on the surface of the photoconductor through a plurality of reflecting mirrors and focusing lenses. Electrostatic latent images are formed on the surface of the photoconductor by the light beams and are developed by an developing agent such as toner supplied through the developing section, whereby the document images are reproduced. The developed images are transferred to the transcript sheet such as a paper sheet supplied from the sheet feeder and fixed by the fixing device. Thereafter, the sheet on which the images have been fixed is ejected. When toner which is thermally fusible is used as a developing agent, fixing is carried out by heating and pressing at the same time.
In an image forming apparatus such as an electronic copying machine employing a photoelectrographic process, an original document loaded on a document table is covered with a document cover. The portion of light beams is emitted from an illumination lamp on the outside areas of the original document, whereby the unnecessary electric charges otherwise left on the surface of a light sensitive drum is removed.
When the original document is read or scanned upon opening the document cover, for example, the light beams emitted from the illumination lamp on the portion outside of the original document do not form images on the photoconductive drum. The unnecessary electric charges at the outside portion of the original document are not removed, and the toner attaches to the portion of the copy sheet outside of the original document after development. When the original document size is smaller than the size of the copy sheet (the sheet to be transferred) or when the copy size is reduced even if the original document size is equal to the size of the copy sheet, the outside portion of the output image (copied image) is blackened by the toner. Further, when a very thick original document such as a book is to be copied, the document cover does not fully contact the document table, and an unnecessary black stripe or stripes are formed on the portion of the copy sheet which is outside of the output image, the portion being hereinafter referred as the outside-of-document portion.
The copy on which the unnecessary black stripe or stripes appear is not attractive and is considered poor in quality. In addition, much toner is wasted. The black stripe or stripes are liable to make the toner adhere to fixing rollers, so that the copy sheet is apt to wind around one of the fixing rollers, to cause a jammed copy sheet.
In the conventional copying machine, the erasable region or regions corresponding to the unnecessary black stripe or stripes are automatically determined by the size of the copy sheet and the image size of the original document to be copied. Thus, the erasable region or regions cannot freely be set.
When a copy holding means is opened during the copying operation, in a case where the document size or the copy size is smaller than the size of the copy sheet, the black stripe or stripes are formed o the outside-of-document portion of the copy sheet. The resultant output images are not only poor in quality, developing agent is wasted.